1. Field of the Invention
The present invention relates to lithium cobalt oxide, a method for manufacturing the same, and a nonaqueous electrolyte secondary battery provided with a cathode plate including the lithium cobalt oxide as a cathode active material.
2. Description of the Related Art
In recent years, nonaqueous electrolyte secondary batteries, such as lithium ion secondary batteries, have become in practical use as power sources for small electronic equipment, e.g., laptop personal computers, cellular phones, and video cameras, in accordance with rapid advance of portable and cordless electronic equipment intended for home use.
With respect to these lithium ion secondary batteries, since lithium cobalt oxide is useful as a cathode active material for the lithium ion secondary battery, research has been actively conducted on lithium-based complex oxides, and many proposes have been made regarding compounds, e.g., lithium cobalt oxide, lithium nickel oxide, and lithium manganese oxide, as cathode active materials until now.
Various proposes for improving the performances of those cathode active materials have been made, and many technologies regarding the apparent density, the pressed density, and the like are disclosed as important factors.
For example, a cathode active material has been proposed, wherein the tap density of LipMO2 containing granular composition prepared by firing at least two types of starting materials having different average particle diameters is at least 2.65 g/cm3 (referred to, for example, the front page of Japanese Unexamined Patent Application Publication No. 2001-85009).
Another cathode active material for a nonaqueous electrolyte secondary battery has been proposed, wherein with respect to a cathode active material used for a nonaqueous electrolyte secondary battery including lithium cobalt oxide represented by a formula, LiCoO2. The above-described lithium cobalt oxide is composed of spherical or ellipsoidal secondary particles which have a Feret diameter of projection pattern of 0.1 to 4 μm on a SEM observation basis and in which many small crystalline primary particles have gathered while the primary particles have an average particle diameter of 2 μm or less. The tap density of the above-described lithium cobalt oxide is at least 2.2 g/cm3 (referred to, for example, the front page of Japanese Unexamined Patent Application Publication No. 2001-135313).
Another cathode active material for a nonaqueous electrolyte secondary battery has been proposed, wherein the cathode active material includes lithium cobalt oxide composed of secondary particles in which many fine primary particles of lithium cobalt oxide subsequently represented by a general formula, LiCoO2, have gathered while the secondary particle has many fine gaps capable of being impregnated with an electrolytic solution, and the lithium cobalt oxide has a tap density of at least 2.2 g/cm3 (referred to, for example, the front page of Japanese Unexamined Patent Application Publication No. 2001-155729).
However, any nonaqueous electrolyte secondary battery including the above-described lithium cobalt oxide as a cathode active material does not simultaneously satisfy the discharge capacity and the quick charge-discharge performance under present circumstances. Therefore, various attempts have been conducted. For example, it has been attempted to change the particle diameter and the shape of particle in order to increase the electrode density and, thereby, increase the battery capacity and in order to improve the quick charge-discharge performance. However, satisfactory results have not yet been achieved.